The present invention relates to laser rangefinder receivers and, more particularly, to laser rangefinder receivers employing the time-of-flight measurement method where multiple targets need to be resolved.
Laser rangefinders are well known, and are used to measure distances to targets. Generally, a laser transmitter is used to beam a high intensity pulse of light onto a selected target. The light scattered from (echoed or reflected off of) the target is detected by an optical receiver (or xe2x80x9copto-receiverxe2x80x9d) which is normally located in close proximity to the laser transmitter. By measuring the transit time (time-of-flight) between a transmitted laser pulse and the received echo, the range (distance) to the target can be determined using a time-interval counter.
Targets may be in proximity to natural or man-made camouflage, resulting in multiple, closely-spaced returns (echoes). Such returns have to be distinguished from backscatter due to aerosols, smoke particles, and ground clutter. The dynamic range of return signal amplitudes is large, and depends collectively on target reflectivity, percentage of beam hitting the target, target range, and atmospheric attenuation. False pulses will occur if the receiver is too sensitive. Range errors and/or second pulse masking may be caused by a strong overloading signal. Second pulse masking often results when a strong pulse is followed closely by a weaker pulse. The first, stronger pulse overloads (saturates) the receiver, and the second pulse occurs before the receiver can recover. As a result, the second pulse is not detected.
A typical time-of-flight laser receiver typically comprises a low-noise detector/preamplifier, a differentiating stage, a time-programmed gain amplifier or time programmed threshold, and a comparator with a digital pulse output corresponding to the time of laser firing and the returned echo pulse signal. The comparator threshold may be a fixed value, or it can be set by a measurement of background noise to obtain a constant false-alarm-rate (FAR). Because of the gain needed to amplify a minimum signal, typically 5 or 6 time RMS noise, multiple stages are typically used. DC offsets build up with this gain, and can cause errors at the threshold, leading to false alarms or loss of sensitivity. This problem has been overcome in the past by using capacitive coupling between stages. However, with strong overloads, these interstage coupling capacitors can accumulate charge, resulting in baseline errors upon recovery, which can exacerbate the problem of false alarms and/or loss of sensitivity.
It therefore is a general object of the present invention to provide an improved technique for performing laser rangefinding.
It is an object of this invention to provide laser rangefinder receiver electronics which changes the sensitivity with respect to time, yet allows a close second target to be measured with minimum error.
It is another object of this invention to provide a receiver without additional interstage coupling capacitors, thereby giving improved validity of the laser range measurement, minimizing false alarms, maximizing sensitivity and range accuracy, and providing excellent target discrimination.
According to the invention, a laser rangefinder receiver comprises a photoconductive detector; a high-pass element producing a detector signal; means for establishing a baseline for the detector signal; means for establishing a detection threshold, offset from the baseline by a threshold offset; and means for comparing the detector signal to the threshold. It should be understood that the detector may be external to the receiver, per se. The high-pass element may be a differentiator or a high-pass filter. The means for establishing a baseline may be a low-pass filter. A noise detector measures a noise level associated with the detector signal, and the noise level establishes the threshold offset from the baseline. The noise detector preferably establishes separate measurements of positive peaks and negative peaks, and the threshold offset is determined from a difference between the positive peak and negative peak measurements. Detector sensitivity is varied by initially decreasing detector sensitivity to a first level, then gradually increasing detector sensitivity to a second level after a trigger event. This can be implemented by means for producing a signal offset, means for establishing an initial level of signal offset, and means for gradually changing the signal offset to a second level. The means for producing a signal offset affects the threshold offset, therefore there is provided means for nulling out the effect on the threshold offset of the means for producing the signal offset. A method is also disclosed. A specific circuit embodiment is disclosed.
The inventive technique exhibits improved capability to distinguish between pulsed signals and noise, responds to a second pulse signal closely following a first pulse signal of either greater or less amplitude, and allows the simplicity and fidelity of DC coupling after a single differentiation. All these factors are desirable to provide error-free range measurement to the target of interest.
Other objects, features and advantages of the invention will become apparent in light of the following description thereof.
Reference will be made in detail to preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. The drawings are intended to be illustrative, not limiting. Although the invention will be described in the context of these preferred embodiments, it should be understood that it is not intended to limit the spirit and scope of the invention to these particular embodiments.
Often, similar elements throughout the drawings may be referred to by similar references numerals. For example, the element 199 in a figure (or embodiment) may be similar or analogous in many respects to an element 199A in another figure (or embodiment). Such a relationship, if any, between similar elements in different figures or embodiments will become apparent throughout the specification, including, if applicable, in the claims and abstract. In some cases, similar elements may be referred to with similar numbers in a single drawing. For example, a plurality of elements 199 may be referred to as 199A, 199B, 199B, etc.